In the past funding period, we have shown that there are critical associations between spirochetes and fibrmolytic proteases of the plasminogen activation system. We have shown how these organisms use plasmin to promote their own invasiveness. The Borrelia binds plasminogen on their surface where it is activated to plasmin by the plasminogen activator urokinase (uPA). In turn, uPA can be bound to its cell receptor, the urokinase plasminogen activator receptor (uPAR, CD87). Borrelia burgdorferi can also induce the upregulation of uPAR on the membrane and the release of the soluble isoform into the medium from peripheral blood monocytes, and from monocyte-like human cell lines. Patients with Lyme disease also have elevated levels of soluble uPAR in the serum and in the cerebrospinal fluid. The function of the soluble uPAR in body fluids in disease, particularly infection, is not known. The hypothesis of this competitive renewal application is that soluble uPAR has an effect on the levels of free uPA, leading to a procoagulant state, which is, in turn, a means to control bacterial dissemination. In this manner, soluble uPAR is a molecule with anti infection properties. The Specific Aims of this proposal are: to determine the biochemical nature of the soluble uPAR released from monocytes after exposure to Borrelia in experimental situations; and, to determine the cellular source and biochemical nature of uPAR in human and experimental infection, and to characterize the clinical conditions of uPAR production. The hypothesis will be tested by several biochemical and molecular approaches, and by animal and clinical studies. This proposal embodies extensive experimental evidence that other macromolecules of the PAS, namely, the uPAR (CD87), could also play a critical role in modulating spirochetal infections. There are also two murine models of spirochetal infection can be used effectively in transgenic animals. Furthermore, the availability of transgenic mice for most of the known components of the fibrinolytic system is advancing our knowledge of this system in areas that had not been previously envisioned. For this proposal, we have in vivo correlates (both at the experimental and clinical levels) to the in vitro observations. Specifically, our fully correlated in vitro and in vivo systems will permit translational conclusions within this funding period.